The invention relates to a receiver for receiving a signal modulated on an RF-carrier the receiver, comprising a pair of signal paths arranged in parallel between an RF input and a superposition circuit and including, arranged in succession, a pair of first mixing stages for mixing down the frequency of the received modulated RF-carrier to a low frequency, mixing signals being applied in a phase quadrature relationship to these first mixing stages from an RF-tuning oscillator, a pair of filtering elements for signal selection and a pair of second mixing stages for up-converting the frequency of the selected modulated carrier, mixing signals being applied also in a phase quadrature relationship to these second mixing stages from a fixed LF-oscillator, these second mixing stages being connected to inputs of the superposition circuit, the superposition circuit comprising an adding or a subtracting stage arranged between outputs of the two signal paths and producing a first output signal.
The principle of such a receiver is known from the article "A third method of generation and detection of SSB signals", by D. K. Weaver, published in "Proceedings of the IRE", 1956, Vol. 44, No. 12, pages 1703-1705, and is not limited to the reception of single-side-band signals. The use of this principle for the reception and processing of double-sideband signals is known, from, for example, German Patent Application No. 26 57 170, which has been laid open to public inspection.
Signal processing in receivers of this type generally amounts to the following: a desired modulation signal modulated on an RF-carrier and applied to the two first mixing stages is down-converted to a low frequency range with the aid of a tunable oscillator frequency at or near the center frequency in the spectrum of this RF-signal. In the case of a single-sideband-modulated RF-signal, this results in the base frequency band of the modulation signal being folded around 0 Hz and in the case of a double-sideband-modulated RF-signal, the center frequency corresponding to the carrier frequency and the oscillator frequency being adjusted to this carrier frequency, this results in a frequency conversion of the modulation signal to the base frequency band. In addition, as a result of the mutual phase quadrature relationship between the mixing signals applied to the two first mixing stages, a 90.degree.-phase difference is produced between the low-frequency (LF) modulation signals at the outputs of these mixing stages.
After signal selection in the two filtering elements, which may be of a simple structure and of a comparatively narrow-band nature, the LF phase-quadrature modulation signals are mixed in the two second mixing stages with the phase quadrature mixing signals of the fixed LF-oscillator. This mixing operation results in two sub-signals whose frequency spectra are mutually mirror-inverted relative to the frequency of the fixed LF-oscillator, and the sum of which is available at the output of one second mixing stage and the difference at the output of the other second mixing stage. As this sum and this difference also form the sum and the difference, respectively, of two equal modulation signals which are mutually mirror-inverted around the last-mentioned LF-oscillator frequency, a suitable superpositioning (that is to say mutually adding and subtracting) of this sum and this difference in the superposition circuit has for its result that the modulation signal with the desired frequency spectrum becomes available, modulated on a center frequency which depends on the fixed LF-oscillator frequency, or in a special case as described, for example, in the above-mentioned article by D. K. Weaver, in the baseband. The unwanted modulation signals in the sum and the difference, respectively, which have a frequency spectrum which is mirror-inverted relative to the desired modulation signals, then cancel each other. The desired, or so-called first, output signal at the output of the superposition circuit is thereafter converted to base-band in a processing circuit and reproduced by means of a reproducing device.
However, in practice, deviations occur in the amplitude and phase correspondence of the signals in the two signal paths, which may cause amplitude and phase deviations in the frequency spectrum of the desired modulation signal at the output of the superposition circuit, and also an image interference signal due to an incomplete mutual cancellation or image frequency rejection of the unwanted modulation signals. More specifically, in wide-band applications, such as, for example in T.V-receivers, these deviations result in impermissible interference effects.